Although the proposed studies (I-III) are diversified with respect to both epithelial cells (kidney, intestine, liver, and gill from various vertebrates) and transported substances (polar and non-polar), they all employ a common physiological-morphological approach and share the same long-term goal, i.e., to elucidate the nature and organization of membrane and cytoplasmic mechanisms involved in transcellular movement. The basic rationale of this project is the premise that only a limited number of such transport mechanisms have developed in the course of evolution. Thus, a broadly comparative series of studies, which take advantage of special features in particular tissues and species, can be most productive of fundamental information which, in turn, bears directly on human health problems. For example, sodium chloride transport across the euryhaline fish gill exhibits adaptive changes in both rate and direction which appear to be central to understanding the renal regulation of salt balance and edema in man (I). Similarly, evidence from flounder kidney tubules suggesting active secretory transport of DDA, an organic acid metabolite of DDT, could lead to procedures for reducing the body burden of the parent pesticide in man (II, III). I. Autoradiographic and functional analysis of mucous secretion and ion transport by osmoregulatory organs of fish, i.e., gill, bladder, kidney, intestine, and skin. II. Autoradiographic and functional analysis of individual transport steps for proteins and organic acids in renal tubular and hepatic cells of vertebrates. III. Analysis of interactions of DDT-like pollutants with specific membrane transport systems.